1. Field of the Invention
The present invention relates to a magnetoresistive effect element, and to a magnetic memory. For example, the present invention relates to the shape of a ferromagnetic tunnel junction element.
2. Description of the Related Art
There has been a magnetic memory using a ferromagnetic tunnel junction (MTJ) element as a memory cell. The MTJ element is mainly composed of ferromagnetic layer/insulating layer/ferromagnetic layer, which are successively stacked. A junction resistance to current tunneling the insulating layer to flow is minimum when the magnetization directions of two ferromagnetic layers are parallel to each other and is maximum when the magnetization directions of two ferromagnetic layers are anti-parallel to ach other.
One ferromagnetic layer is called a reference layer whose magnetization direction is fixed. The other ferromagnetic layer is called a recording layer whose magnetization direction changes. Parallel or anti-parallel state of the magnetization direction of the reference and storage layers is made to correspond to binary information, and thereby, information is stored. Write current flowing a write line that is provided around a memory cell generates a magnetic field to switch the magnetization of the recording layer to write information.
An advance in the scale-down of memory cell in order to improve the integration of magnetic memories reduces a ferromagnetic material forming the memory cell. Smaller ferromagnetic material generally has larger coercivity. Larger coercivity requires greater switching magnetic filed for switching the magnetization direction of a recording layer. Thus, a write current required for generating a switching magnetic filed becomes large; as a result, power consumption is increased. Therefore, in order to realize lower power consumption, it is important to reduce the coercivity of the small magnetic material.
Meanwhile, a parameter called a thermal fluctuation constant exists as an index of memory cell stably holding information for long term. The thermal fluctuation appears to be proportional to volume and coercivity in general. Therefore, high thermal stability is required to hold information for long term; nevertheless, reduced power consumption degrades the thermal stability.
Moreover, an edge domain gives an influence to a switching magnetic field. The edge domain is a special magnetic domain and appears around an edge of a rectangular micro ferromagnetic material whose width of the shorter axis is, for example, less than sub-micron from several microns. The edge domain is disclosed in the document, J. App. Phys. 81, 5471 (1997)(J. App. Phys. 81, 5471 (1997)), for example.
The edge domain is generated because the magnetization is given along the shorter side of rectangle and form a spirally rotating pattern in order to reduce anti-magnetic energy in the shorter side. The magnetization is generated in the direction different from the middle portion in both edge portions of the magnetic material although it is generated in the direction according to magnetic anisotropy in the middle portion of the magnetic material. During spin switching in the rectangular ferromagnetic material, the edge domain is known to grow its size. Thus, the edge domain serves to increase a switching magnetic filed.
The edge domain is sensitive to the shape of the ferromagnetic material. For this reason, the use of an elliptic ferromagnetic material is proposed in U.S. Pat. No. 5,757,695.
Moreover, U.S. Pat. No. 5,748,524 and JPN. PAT. APPLN. KOKAI Publication No. 2000-100153 disclose that the edge domain is fixed to prevent a complicate change of magnetic structure from occurring in spin switching as much as possible. The technique can control magnetization behavior in spin switching but cannot reduce a switching magnetic filed. Moreover, another structure must be added to fix the edge domain; therefore, this is not suitable for achieving high density.